Power electronics technology is the field of engineering which deals with the conversion, control and conditioning of electric power utilizing electronic power devices. Electronic power can be controlled as to its basic form (alternating or direct current) its effective voltage or current, its frequency and its power. In turn, the control of electric power is frequently used to maintain or achieve or regulation of some other non-electrical factors such as the speed of a motor, the temperature of a heating device or the measurement or generation of light. Control of electric power is likewise used in logic circuits which form the basis for the operation, control and applications of electronic computers.
In current technology, solid state devices have achieved nearly universal acceptance in most applications for such conversion, control and conditioning of electrical power, as well as in the computer industry. Semiconductor devices exhibit greater reliability, faster speed, higher efficiency, smaller size and often lower cost.
One basic type of solid state device is the rectifier diode. As is know to those familiar with electronic devices, diodes are the simplest kind of semiconductor device and the most straightforward use of diodes is rectification; i.e. the conversion of alternating current to direct current. Diodes act as one-way barriers to the passage of electrons in that they permit electrons to flow in one direction but bars their passage in the other direction. Thus, diodes are useful as switching devices. In electronics terminology, a diode is said to pass current in the forward direction ("forward bias") and block current in the reverse direction ("reverse bias"). The characteristics of any given diode can generally be determined by the relationship between the voltage (V) applied to the diode and the current (I).
As is further known to those familiar with electronic devices, however, current is never always completely blocked in the reverse direction. When a reverse voltage (V.sub.R) is applied, a small amount of reverse current (I.sub.R) will flow through the diode. The reverse current of any diode is therefore defined as the amount of current which will flow at a given reverse voltage. Under conditions of reverse voltage, however, an amount of voltage will finally be reached where the rectifying or electron blocking capability of the diode begins to break down completely. This point is called the reverse breakdown voltage (V.sub.BR) and represents the point at which the diode undergoes avalanche multiplication or tunneling of carriers in the depletion region. At this voltage, current will increase dramatically, and if not limited, will qenerally destroy the diode because of the high wattage and destructive heat that results.
In general, the performance of a diode can thus be characterized using five basic characteristics:
Forward current (I.sub.F), the amount of current the diode can handle without burning up;
Forward voltage (V.sub.F), the voltage level necessary to produce the desired forward current level;
Reverse current (I.sub.R), the amount of current that will leak through the diode at a given reverse voltage;
Reverse breakdown voltage (V.sub.BR), the reverse voltage beyond which the flow of reverse current begins to rise very rapidly; and
Reverse recovery time (t.sub.rr), the time it takes the diode to recover from forward conduction and begin to again block reverse current.
Reverse recovery time is an important characteristic in defining the frequency of alternating current that a given diode or rectifier can handle. The higher the frequency of alternating current which is imposed on the diode, the more quickly the diode must respond in order to rectify this current.
The characteristics of any such device, of course, depend to a great degree upon the material from which the semiconductor device is formed. Different materials have different inherent electronic characteristics and capabilities, and for any given semiconductor material the quality of the devices that can be manufactured will generally depend upon the crystal structure, purity and appropriate doping that can be accomplished with such materials.
Silicon carbide has long been a candidate material for use in the manufacture of such semiconductor electronic devices. Silicon carbide has a number of characteristics which make it theoretically advantageous for such uses. These include a wide band gap, a high thermal conductivity, a low dielectric constant, a high saturated electron drift velocity, a high breakdown electric field, a low minority carrier lifetime, and a high dissociation temperature. Taken together, these properties indicate that semiconductor devices formed from silicon carbide should be operable at much higher temperatures than devices made from other semiconductors, as well as at higher speeds and higher power levels.
Nevertheless, rectifying diodes and other semiconductor electronic devices made from silicon carbide have yet to make a viable appearance in any circumstances other than laboratory research and have yet to reach their commercial potential. This lack of success results, at least partially, from the difficulty encountered in working with silicon carbide. It is an extremely hard material, often used as an abrasive. It often must be worked at extremely high temperatures under which other materials cannot be worked, and from a semiconductor standpoint, crystallizes in well over 150 polytypes, many of which are separated by rather small thermodynamic differences. For these latter reasons, production of monocrystalline thin films of silicon carbide that are necessary for certain devices, and production of large single crystals of silicon carbide which are useful as substrate material and for other applications, have remained elusive goals. Additionally, certain doping techniques which have been successfully developed for other materials have proved unsuccessful when used in connection with silicon carbide. Finally, p-n junctions appropriate for rectifying purposes have yet to make a successful practical and commercial appearance.
Recently, however, a number of developments have occurred which have successfully accomplished both single crystal bulk and thin film growth of silicon carbide. These are included in several co-pending patent applications which have been assigned to the assignee of the present invention, and the contents of which are incorporated entirely herein by reference. These include: Davis et al, "Growth of Beta-SIC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 113,921 Filed Oct. 26 1987; now U.S. Pat. No. 4,912,063 and Davis et al, "Homoepitaxial Growth of Alpha-SIC This Films and Semiconductor Devices Fabricated Thereon," Ser. No, 113,573, Filed Oct. 26, 1987; , now U.S. Pat. No. 4,912,064 both of which are incorporated entirely herein by reference; Palmour, "Dry Etching of Silicon Carbide," Ser. No. 116,467, Filed Nov. 3, 1987; now U.S Pat. No. 4,865,685 and Davis et al, "Sublimation of Silicon Carbide to Produce Large, Device Quality Single Crystals of Silicon Carbide," Ser. No. 113,565, Filed Oct. 26, 1987 now U.S. Pat. No. 4,866,065.
The production of an appropriate, defined p-n junction of performance quality is a fundamental step in fabricating a rectifying diodes. Accordingly, given the theoretical advantages of silicon carbide and the necessity of producing junctions to develop the devices, including rectifiers, there has been significant interest in methods of producing such junctions in silicon carbide. Most of these efforts have developed methods of producing what may be referred to as "fused" junctions. In such junctions, alternating regions of p-type and n-type silicon carbide are formed in contact with one another to form the p-n junction. Typical techniques have included melting a dopant metal directly on the surface of silicon carbide so that some of the dopant dissolves into the silicon carbide to produce an oppositely doped region, the border of which forms the p-n junction. Others use separately formed portions of p-type and n-type silicon carbide and fuse them to one another using various processes to form the p-n junction Other techniques attempt to encourage epitaxial growth of p or n-type silicon carbide upon the substrate of silicon carbide having the opposite conductivity type. A number of other methods include solvent based techniques. In a related copending application assigned to the assignee of the present invention, successful use of ion implantation techniques has been demonstrated to result in appropriate junctions; "Implantation and Electronical Activation of Dopants Into Monocrystalline Silicon Carbide," Ser. No. 113,561, Filed Oct. 26, 1987.
Similarly, a number of attempts have been made to produce successful rectifying diodes in silicon carbide. These include rectifying contacts (Schottky Diodes) between particular metals to either n or p-type silicon carbide, the fusion techniques described previously, electric arc or sputtering techniques for depositing a metal rectifying contact to silicon carbide, building stepped transitions between diode materials including silicon carbide, and using a thin layer of glassy amorphous material as an active layer in a rectifying junction device in silicon carbide.
More recently, and as set forth earlier herein, silicon carbide growth techniques using chemical vapor deposition (CVD) to produce high quality epitaxial layers of silicon carbide on both silicon and silicon carbide substrates have been demonstrated. Using these techniques, yet other researchers have attempted to successfully form rectifying diodes in silicon carbide. For example, Kuroda, et al. "Stepped Controlled VPE Growth of SIC Single Crystals at Low Temperatures", Extended Abstracts of The 19th Conference on Solid State Devices and Materials, Tokyo, 1987, pages 227-230, described their attempts to produce diodes in silicon carbide by forming adjacent p and n epitaxial layers using chemical vapor deposition techniques. In their work, Kuroda, et al. report production of a diode that has a reverse breakdown voltage of about 100 volts and a forward current of about 400 microamps at about 3 volts. Kuroda does not report any reverse recovery time, an important characteristic in a rectifying diode, and produces a graded p-n junction. As known to those familiar with such devices, the characteristics of the junction will essentially define the characteristics of the entire device. A graded junction is one in which a gradient exist between the opposite types of charge carriers (electrons and holes) which can extend for some distance across the junction including distances as great as one or more microns. Conversely, an abrupt junction exhibits rapid change from one carrier type to the other. As is further known to those familiar with such devices, one technique for determining whether a junction is graded or abrupt is the use of the measurement of the capacitance of the diode in relationship to the voltage. Generally speaking, when capacitance is plotted directly against voltage, a nonlinear relationship exists. A linear relationship will exist, however, between the reciprocal of the capacitance squared or the reciprocal of the capacitance cubed. If the relationship between the reciprocal of the capacitance squared and the voltage is nonlinear, the diode may be characterized as having an abrupt junction. Alternatively, if the relationship between the reciprocal of capacitance cubed plotted against voltage is linear, the junction can be characterized as graded. As set forth by Kuroda, the diode produced using his techniques exhibits such a graded junction.
Additionally, Kuroda's diode operates in the microamp range, a smaller amount of current capacity than would otherwise be useful in most power requirement applications. Finally, although one desirable characteristic of silicon carbide is its ability to produce blue light, for a rectifying diode a more important characteristic is operation at high temperatures and Kuroda fails to demonstrate any successful diode application at other than room temperature.
Accordingly, it is a object of the present invention to provide a rectifying diode formed in silicon carbide which can operate at high frequency, at high reverse voltage, and at high temperatures, with an abrupt junction and low forward resistance.